Several distinct fibroblast subtypes reside within the interstitium of the lung. Activation of one or more of these fibroblast subtypes likely contributes to the pathogenesis of pulmonary fibrosis. In order to examine fibroblast heterogeneity, we will employ in our studies two fibroblast major subtypes which we have isolated from the neonate rat lung. We plan to examine the effects of important effector molecules such as platelet-derived growth factor and transforming growth factor-beta on these fibroblast subtypes. These substances are likely generated during inflammatory reactions following acute lung injury. In addition, we will evaluate the effects of defined matrix components and cell-cell interactions on the two fibroblasts subtypes. The effect of normal or bleomycin-injuried alveolar type 2 cells on the fibroblast subtypes will be assessed. We will determined whether either of the fibroblast subtypes is selectively stimulated by these experimental manipulations. A characteristic feature of the early fibrotic lesion is the appearance of the myofibroblast. The myofibroblast contains peripheral actin bundles and is associated with the deposition of type III collagen. The mechanisms which control the expression of the myofibroblast phenotype are unclear. We will examine the hypothesis that one of the fibroblast subtypes is modified to develop the myofibroblast phenotype by inflammatory mediators, defined matrix components or fibroblast-epithelial interactions. The distinct fibroblast subtypes will be examined morphologically and the synthesis of actin, fibronectin and collagen will be determined. The types of collagen synthesized will be characterized and the expression of mRNA for types I and III collagen synthesized will be assessed using cDNA probes. We will determine whether myofibroblasts isolated from bleomycintreated lungs resemble either of the fibroblast populations studied above. We expect that these studies will provide new insights into the pathogenesis of pulmonary interstitial fibrosis.